A standard motor-vehicle central lock system has a plurality of door latches each provided with an actuator that can move the latch between a locked position in which manual actuation of the latch is ineffective to open the respective door and an unlocked position in which such actuation can open the door. The same or another actuator can also in some systems actually unlatch the door so that, when it operates, the door springs open.
U.S. Pat. No. 6,075,294 of van den Boom describes a motor-vehicle door latch where a capacitor is formed between an element mounted on a door handle and another element carried on the door and juxtaposed across an open gap with the door-handle element. Thus these two capacitor elements form a capacitor of an impedance that is mainly determined by the size of the air gap between the two elements and their areas. When a user also carrying a radio-frequency interrogable transponder or data carrier grabs the handle and thus puts his or her fingers between the two elements, this capacitance changes and a control circuit initiates a query of the transponder. Presuming the transponder holds the appropriate code, the latch is unlocked and the user can open the door.
Such a keyless entry system is extremely convenient in that it allows a user carrying the transponder, typically incorporated in the ignition key or carried on a key chain, to open the vehicle door without having to take any special action. The door is unlocked as the user grasps the handle, but will not unlock for anyone except a person carrying an appropriately encoded transponder.
The above-described system works by forming a fixed-impedance capacitor whose capacitance is changed by interposition of the user's hand between the elements of the capacitor. Such interposition changes the dielectric constant between the two elements and/or effectively decreases the spacing between them by adding the semiconductive hand to one of the elements. It is a simple manner to exploit this change in capacitance to operate the lock system in accordance, for example, with principles described in U.S. Pat. No. 5,730,165 of Philipp.
A problem with this system is that it requires the user to actually grasp the door handle and insert his or her fingers between the two capacitor elements typically mounted on the inside face of the handle and the confronting outside face of the door. If the system does not function very quickly, the user might have to wait briefly while the door is unlocked.
U.S. Pat. No. 6,002,341 of Ohta describes a door-latch actuator that has a contact electrode connected to a latch controller and serving to determine when the door handle is touched by the user. A remote control is provided with an electrode. When the user carrying the remote control touches the door handle a capacitive connection is made between the contact electrode in the door handle of the vehicle and the electrode in the remote control. Thus the body of the user functions as a dielectric between the two electrodes, one on the handle and the other in the remote control. Since the dielectric impedance between the two electrodes changes as the user approaches the door handle, the capacitance changes and this change can be exploited to operate the door latch.
Another system described in U.S. Pat. No. 5,880,538 of Schulz describes a capacitive proximity switch serving to unlock a door latch. This arrangement monitors the capacitance between an electrode and ground and operates the door latch only when the rate of change of this capacitance is greater than a predetermined lower limit. The system works together with a transponder carried by the user, so that the combination of a predetermined change rate of the capacitance and the presence of the transponder makes the door unlatch.
Finally European 0,954,098 of Pavatich relates to a switching arrangement having a conductor that is part of a variable capacitor whose capacitance is related to the proximity of a user. A detector determines changes in this capacitance and produces an output when it indicates that a person is nearby. This detector has an oscillator that is coupled with the conductor and a phase comparator. Changes in capacitance result thus in a variation in the oscillation frequency of the oscillator and can therefore be detected, as in above-cited U.S. Pat. No. 4,871,204.